If you use a dedicated gateway, you can specify either an IP address that belongs to the same subnet where the gateway is deployed, or the private address of a local data center connected to the gateway through Direct Connect.
Unsupported network segments:
If you use the shared gateway on the old console, you cannot specify the backend address as a subnet IP address. For a backend service deployed on multiple ECSs that are located in the same region but are not bound with any EIPs, create a VPC channel and associate the ECSs with it.